1. Field of the Invention
The present invention relates to an impact device having an impact plunger operable by fluid pressure, and particularly to an impact device suitable for a hydraulic breaker or a rock drill for crushing a rock base.
2. Description of the Prior Art
Conventionally, an impact device is mounted on an arm of a construction machine such as a hydraulic excavator and is connected to a hydraulic pump which is normally provided on the construction machine, so that the impact device is driven by the hydraulic pump.
However, the amount of discharge of hydraulic fluid varies substantially with the capacity of the hydraulic pump provided on the construction machines such as hydraulic excavators. Thus, the performance of the impact device depends on the amount of discharge of hydraulic fluid from the hydraulic pump of the construction machine on which the impact device is mounted.
Further, pressure-receiving surfaces formed on a plunger of the conventional impact device for impact movement of the plunger have the same area with each other. Therefore, if the impact force is to be increased, the stroke of movement of the plunger must be determined to have a greater length, resulting in that the number of impacts is reduced. Furthermore, even if the amount of discharge of the hydraulic pump is larger than the amount of consumption of the hydraulic fluid in the impact device, the impact force cannot be increased since the capacity of the hydraulic pump cannot be fully utilized. Additionally, in this case, the excessive hydraulic fluid must be returned to a reservoir tank via a pressure control valve, etc., resulting in that the hydraulic fluid is heated or stained and that an excessive load is applied to the hydraulic pump. On the other hand, if the capacity of the hydraulic pump is too small as compared with the amount of possible consumption of the impact device, the operational pressure is reduced, resulting in that the performance of the impact device is degraded.